This application is related to U.S. Pat. Nos. 5,289,692, 5,477,701 and 5,522,231; the entire disclosures of which are fully incorporated herein by reference. These patents are referred to herein collectively as the Low Side patents.
The invention relates generally to heat transfer and refrigeration control systems. More particularly, the invention relates to apparatus and methods for detecting characteristics and for controlling mass flow of the working fluid in such systems.
The basic building blocks of all refrigeration and heat transfer systems are well known and include a compressor, a condenser, an expansion means and an evaporator, all of which are connected in a fluid circuit having a working fluid such as halogen containing working fluids such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs), and so forth. In an automotive or truck air conditioning system, for example, the working fluid or refrigerant is typically in heat exchange with the vehicle compartment ambient air by means of the evaporator. The liquid refrigerant turns to gas as it passes through the evaporator or endothermic heat exchanger thus absorbing heat from the ambient air. The working fluid leaving the evaporator, preferably is in an all gaseous state, and is drawn into the compressor through a suction line. The compressor increases the gas pressure and the gas then passes through the condenser or exothermic heat exchanger where it is cooled back to a liquid state but is still under high pressure. The liquid high pressure working fluid is then passed through the expansion means, such as an expansion valve, wherein the fluid pressure is adiabatically decreased prior to re-entering the evaporator.
Over the years, many different types of control mechanisms and monitoring devices have been used to regulate the operation of heat transfer or refrigeration systems. One of the more important functions required of a heat transfer control system is to monitor and control the low pressure state of the working fluid in the suction line near the outlet of the evaporator or at the inlet to the compressor. Usually, the systems are designed to operate with the working fluid in a superheated state at the outlet of the evaporator. This is important for many reasons, particularly to maximize cooling from the working fluid flow, and to protect the compressor from receiving liquid working fluid and/or a loss of lubricant.
A known technique for controlling the working fluid state is to maintain a minimum superheat state in the vaporous working fluid exiting the evaporator. The superheat is usually maintained in the range of 5 to 10 degrees fahrenheit. In some systems, the superheat is regulated by monitoring the evaporator inlet and outlet temperatures of the working fluid and controlling the flow with the expansion valve so that the temperature difference is near a preset value or range. Other approaches include the use of pressure and temperature sensors on the outlet side of the evaporator to measure the actual temperature and pressure characteristics of the working fluid based on the thermodynamic properties of the working fluid. Still another approach is the use of charged bulb sensors. From a heat transfer efficiency standpoint, it is desirable, of course, to maintain a low superheat which is difficult with the aforementioned sensors and controls.
Although these known approaches for regulating superheat can work, they tend to exhibit inaccurate control. One reason is that in the evaporator the liquid and gas phases are not in thermal equilibrium. The droplets of gas are boiling because heat is being transferred to the droplets from the gas phase. In order for this to take place, the gas must be hotter than the liquid, which makes conventional superheat measurement difficult.
Some heat transfer systems are designed to operate "wet", in other words with an expected quality less than 100% at the evaporator outlet. Known apparatus for detecting working fluid conditions and states such as percent quality and superheat typically operate best under one of these states or the other, but not both. For example, a quality detector that is sensitive to liquid droplets in the working fluid may not operate accurately to detect superheat conditions. Detectors that operate to measure superheat may not operate accurately to detect working fluid states in which the quality is less than 100%.
Another significant problem with low superheat control systems is that pressure drop of the working fluid through the evaporator can change over a large range, particularly for heat transfer systems that operate under varying loads and other dynamic operating conditions. Substantial pressure changes reduce the effectiveness of conventional superheat based control systems. Therefore, superheat as measured by temperature differentials across the evaporator is a poor control mechanism for regulating the state of the working fluid at the outlet of the evaporator in dynamic systems that exhibit significant changes in evaporator pressure.
The apparatus and methods, for detecting and controlling quality in a working fluid, that are described in the above-incorporated Low Side patents, may not always provide adequate control when systems operate with the working fluid in higher superheated states, such as, for example, greater than 5.degree. F. For example, a quality sensor that is used for determining thermal conductance between the sensor and the working fluid droplets is not always effective under higher superheat conditions because of the relative absence of liquid droplets in the working fluid at the sensor. It has also been found that a self-heated thermistor, when used as a quality sensor, produces readings that are dependent on the thermal load conditions of the evaporator. Although the Low Side sensor techniques in the referenced patents are useful in many applications, it is desired to improve the effectiveness of such sensors under conditions of higher superheat and/or substantial load variations.
Accordingly, the objectives exist for economical, reliable and accurate apparatus and methods for detecting state characteristics of a working fluid in a heat transfer system, particularly as those characteristics relate to detecting and controlling quality and superheat states of the working fluid on the outlet side of the evaporator.